The present invention relates to solvent-free, thermosetting molding materials based on polyurethane which are stable in storage and contain a fibrous reinforcing material, and a process for their production.
Fiber-reinforced moldings may be produced according to prior art processes using many different synthetic polymers. Glass fibers have proved to be a particularly suitable reinforcing material. One example of such processes is the production of semifinished products which are stable in storage (sheet moulding compound, "prepregs", bulk moulding compounds) from resins containing unsaturated polyesters (abbreviated hereafter as "UP-resins") and glass fibers, which semifinished products may be molded using heated presses and cured into moldings of a very high strength and rigidity. However, this process has several disadvantages. The monomers (e.g., styrene) containing vinyl groups and usually used as a solvent for the UP-resins, lead during the radically initiated polymerization to a very high density of cross-linking which causes extreme brittleness and very high notched bar sensitivity of the molding. Moreover, the presence of solvents complicates processing since complicated procedures have to be carried out to remove the solvent vapors by suction, and antiexplosion processing installations are required.
In order to produce the semifinished product, the UP-resin must be thickened so that, in the subsequent heating/pressing process, the reinforcing fibers are uniformly transported to any place of the molding. For this purpose, mostly small quantities of an alkaline earth metal oxide or hydroxide are added to the resin, which initiate a protracted maturation process which usually lasts from 7 to 21 days and must be carried out in a so-called "maturation case" at a slightly elevated temperature. This intermediate treatment requires a considerable consumption of time and capital and impairs the efficiency of the process.
It is also known to reinforce polyurethane elastomers using fibrous materials. Thus, for example, glass fiber-reinforced polyurethane moldings may be produced by the injection molding or reaction injection molding process. However, the mechanical characteristics, in particular the flexural strength, of such moldings may only be increased to a certain extent, as the length of the fibrous material may not exceed from 1 to 6 mm approximately for processing reasons. A particular disadvantage is that, due to the limited fiber length, the thermal expansion coefficient of the reinforced polyurethane elastomers is still many times greater than the expansion coefficient of steel.
A process for the production of high molecular weight, cross-linked plastics is described in German Pat. No. 968,566, in which an intermediate product is produced initially from a polyester having hydroxyl groups, a glycol and an excess of diisocyanate. The intermediate product is then used to prepare semifinished products which are stable in storage, by reaction with an excess of a diisocyanate containing uretdione groups. These semifinished products may finally be molded and cured into elastic moldings by the effect of temperature.
The polyurethane elastomers described in German Pat. No. 968,566 do indeed have a high elasticity and tenacity, but their hardness and rigidity is insufficient for many uses. The use of a fibrous reinforcing material is not mentioned in German Pat. No. 968,566. In principle, glass fibers of a length of greater than 6 mm may indeed be incorporated in the intermediate product described above; however, the high viscosity of the intermediate product would necessitate the use of rolling mills or kneaders for mixing in the fibrous material and the uretdione diisocyanate required for cross-linking. These types of apparatus are conventional in rubber processing. However, when working in the fibrous material in this manner, such high shearing forces occur, under the conditions of the process, that the individual fibers become pulverized into fractions of their initial length so that the required effects of reinforcement on one hand and reduction of the thermal expansion coefficient on the other hand cannot be fully obtained.
An object of the invention is to provide new solvent-free, storage stable molding materials which may be cured into moldings having high rigidity, desirable impact strength and high dimensional stability as a result of pressing at elevated temperature.
This object is achieved using the molding materials provided according to the invention.
A further object of the invention is to provide a process using the molding materials to produce fiber-reinforced moldings having high rigidity, desirable impact strength and high dimensional stability and to produce them in economically acceptable periods of time, i.e., without protracted intermediate treatment of the preliminary molding. This object is achieved by the present process.